This invention relates to a variable speed control apparatus for controlling induction motors by the transvector control method. Transvector control systems, which control motors without using a speed sensor, are known. For example, motor controllers of such design are disclosed in Japanese Laid Open Patent Publications No. S64-8896, and No. H0 1-198292. In such systems, control is effected using a primary angular frequency for detecting a position of a magnetic flux axis based on the induced voltage of the motor 2.
Referring to FIGS. 3, 15, and 16, the operation of the primary angular frequency operating means 200 ' of a variable speed control according to the prior art will be briefly explained. FIG. 3 is a vector diagram of an induced voltage vector E. FIG. 15 is a block diagram of the variable speed control of the prior art, which is similar to the variable speed control apparatuses described in the above laid open publications. FIG. 16 is a block circuit diagram showing a primary angular frequency operating means 200 ' of FIG. 15. An absolute value operating circuit 53 calculates an absolute value .vertline.E.sub.T .vertline. of a T axis component E.sub.T (T axis induced voltage) of an induced voltage vector E. An M axis is an axis of a rotating coordinate system (M,T axes) lying in a direction of magnetic flux, and the T axis is an axis perpendicular to the M axis. The absolute value .vertline.E.sub.T .vertline. of the T axis component of voltage vector E is fed to a divider 28. The divider 28 calculates a primary angular frequency by dividing the absolute value .vertline.E.sub.T .vertline. of the T axis component of voltage vector E by a magnetic flux command value o.sub.2 *.
As FIG. 3 shows, an M axis component E.sub.M (M axis induced voltage) of induced voltage vector E has a non-zero magnitude when a phase angle command value .theta.*, differs from an actual phase angle value .theta. of motor 2. Phase angle command value .theta.* is generated by an integrator 102 of FIG. 15. To eliminate this difference, thereby insuring that the phase angle command value .theta.* represents the actual phase angle value .theta., primary angular frequency operating means 200 ' regulates its output to maintain M axis component E.sub.M, of induced voltage vector E, at zero. The regulation accomplished by a regulator 52. The M axis component E.sub.M, of induced voltage vector, E is applied to an input of regulating means 52. The output of output of regulating means 52 is applied to a negative input of an adder 55. The output of divider 28 is applied to the positive input of adder 55 to generate an absolute value of a primary angular frequency command value .vertline..omega..sub.1 *.vertline.. Regulating means 52 may be a proportional (P) controller or proportional-integral (P-I) controller. This control operation adjusts an absolute value of a primary angular frequency command value .vertline..omega..sub.1 *.vertline. so that the phase angle command value .theta.* coincides with the actual phase angle .theta.. The final primary angular frequency command value .omega..sub.1 * is generated by detecting the polarity of E.sub.T in a polarity detecting circuit 54, and multiplying the detected polarity (sign) by the absolute value of the primary angular frequency command value .vertline..omega..sub.1 *.vertline. in a multiplier 51.
Primary angular frequency command value .omega..sub.1 *, output by primary angular frequency operating means 200', is applied to an input of an integrator 102 of the prior art variable speed control (FIG. 15). Integrator 102, a means for generating a magnetic flux position, generates the phase angle command value .theta.*. The phase angle command value .theta.* is used for voltage and current vector rotation in vector rotators 11 and 24, and for coordinate transformation, in a coordinate transformer 8. An adder 202 calculates an estimated speed value .omega..sub.r by subtracting a slip frequency command value .omega..sub.s *, calculated in a slip frequency operator 101, from .omega..sub.1 *. (Note that the symbol indicates estimated values in the figures, however, this symbol does not appear in the specification to denote the corresponding terms.) The estimated speed value .omega..sub.r is used for speed control of an induction motor 2. Slip frequency command value .omega..sub.s * is generated by slip frequency operator 101 according to the following equation: ##EQU1## where R.sub.2 is a resistance.
An actual magnetizing current value I.sub.M and an actual torque current value I.sub.T are generated as follows. A primary current of induction motor 2 is detected by a current detector 203, and resolved in a three-phase/two-phase transformer 12 into two-phase current components i.sub..alpha., i.sub..beta. of the stator coordinate system. Two-phase values i.sub..alpha., i.sub..beta. are further transformed in a vector rotator 11 to the actual magnetizing and torque current values I.sub.M and I.sub.T on the rotating coordinate (M-T axes) defined with respect to the axis of magnetic flux.
A magnetic flux regulator 4 generates a magnetizing current command value I.sub.M * responsively to the magnetic flux command value o.sub.2 *. A speed regulator 5 generates a torque current command value I.sub.T * responsively to a speed command value .omega..sub.r * and the estimated speed value .omega..sub.r. Magnetic flux regulator 4 and speed regulator 5 may be proportional (P) or proportional-integral (P-I) controllers. Speed regulator 5 may be a series compensated P or P-I controller, to which a difference of speed command value .omega..sub.r * and estimated speed value .omega..sub.r, generated by an adder (not shown), is applied as an input.
Magnetizing current command value I.sub.M * and torque current command value I.sub.T * are applied to inputs of a current regulator 6. Current regulator 6 generates an M component V.sub.M * (magnetizing voltage command value) and a T component V.sub.T * (torque voltage command value) of a primary voltage command value from the command values I.sub.M *, I.sub.T * and the actual values I.sub.M, I.sub.T. Current regulator 6 may be a pair of independent series compensated P or P-I controllers. In such case, a signal from an adder (not shown), equal to the difference between I.sub.M * and I.sub.M, is fed to a first of the controllers and used to generate V.sub.M *. A signal from another adder (not shown), equal to the difference between I.sub.T * and I.sub.T, is fed to a second of the controllers and used to generate V.sub.T *.
The voltage command values V.sub.M *, V.sub.T * are transformed by coordinate transforming circuit 8 to generate two-phase values v.sub..alpha. *, v.sub..beta. * based on phase angle command value .theta.* supplied by integrator 102. Coordinate transforming circuit 8 rotates vector [V.sub.M *, V.sub.T *] to obtain vector [v.sub..alpha. *, v.sub..beta. *] according to equation b: ##EQU2## Two-phase values v.sub..alpha. *, v.sub..beta. * are further converted in a pulse generating circuit 9 to drive pulses (as described in connection with FIG. 1 ) for driving a PWM inverter 1.
A voltage detector 20 and a three-phase/two-phase transformer 21 are used to generate two-phase values v.sub..alpha. and v.sub..beta.. The vector rotator 24 generates magnetizing and torque voltages V.sub.M and V.sub.T from two-phase values v.sub..alpha. and v.sub..beta.. Induced voltage operating circuit 22 generates the M and T axis components of the induced voltage E.sub.M and E.sub.T according to equation c: EQU E.sub.M =V.sub.M -(R.sub.1 +pL.sigma.).multidot.I.sub.M +.omega..sub.1 .multidot.L.sigma..multidot.I.sub.T EQU E.sub.T =V.sub.T -(R.sub.1 +pL.sigma.).multidot.I.sub.T +.omega..sub.1 .multidot.L.sigma..multidot.I.sub.M (c)